As for a fluorescent display device having a chip in glass structure (hereinafter, referred to as CIG structure) in which an IC chip, i.e., integrated circuit devices on a semiconductor substrate, is fixed on a glass anode board serving as a part of an envelope, there is known a device which protects an IC chip from external light or the like with an IC cover formed of, e.g., 426 alloy.
However, in case of using the IC cover, there is a limit in narrowing a gap between a filament and an anode. Particularly, in case of a low voltage operation, it is difficult to narrow an FH (distance between the filament and the anode), thereby making it difficult to attain a high level of brightness.
Further, in case where the IC cover is installed, conductive materials generated during a welding splash process or the like adhere between a bonding wire and its neighboring wire to thereby cause a short-circuit therebetween. As a result, there may be generated an IC malfunction or a display fault. In order to solve the above problems, Japanese Patent No. 3553868 provides a technique for coating an IC chip covered by an IC cover or a bonding wire portion with a coating material.
As shown in FIG. 9, in a fluorescent display device 50 disclosed in Japanese Patent No. 3553868, an IC chip 71 is fixed on a substrate 51 by a die bonding paste via a black insulating film 52, and a first electrode on the substrate 51 and a second electrode on the IC chip 71 are connected to each other by a wire. Further, an insulating layer 72 is formed to cover the first and the second electrode and the wire. After a lead frame as a metal element for unifying a filament support 53 or the like is disposed above the IC chip 71, the substrate 51 and glass vessels 60 and 61 are sealed by fritted glasses 62A and 62B, thereby forming an envelope. Furthermore, the second electrode of the IC chip 71 and the wire are coated with a coating material 74 containing insulating particles 73, wherein a mean particle size thereof is smaller than or equal to one third of a pitch between neighboring wires.
In Japanese Patent No. 3553868, the coating material 74 for coating the IC chip and its neighboring portion is obtained by dispersing Al2O3 serving as insulating particles 73 into a solution where Al alkoxide as a solute is dissolved in alcohol. Here, a mean particle size of Al2O3 is known to be 1 μm, and a weight ratio of Al2O3 to Al alkoxide is set to be 0.5. In addition, by heating the coating material 74 under an atmosphere of 480° C., alcohol contents therein are evaporated and, then, an insulating layer 72 of Al2O3 is formed from an Al alkoxide by a baking process. Since the insulating particles 73 made of Al2O3 are not shrunk by the heating, the coating material 74 including the insulating layer 72 and the insulating particles 73 is less shrunk than the case when it is exclusively composed of a coating insulating layer during the baking process, thereby reducing a stress applied to the wire. Accordingly, there is disclosed a technique for preventing the wire itself from being cut off or preventing the connection between the wire and the electrode from being cut.
Alternatively, the coating material 74 may be obtained by dispersing SiO2 serving as insulating particles into a solution where polyimide resin as a solute is dissolved in dimethylacetamide. Here, a mean particle size of SiO2 is known to be 5 μm, and a weight ratio of SiO2 to polyimide resin is set to be 1 (that is, a weight ratio of SiO2 to polyimide resin is set to be 1:1). If a heating temperature is lower than 400° C., the coating material 74 has no degassing effect. Therefore, there is disclosed a technique for generating the insulating layer 72 formed of a polyimide film by performing the baking process at a temperature ranging from 400° C. to 500° C. such that the insulating particles 73 of SiO2 will be made to be adhered between wires and between the wire and the black insulating film 52 with the use of the insulating layer 72.
As for a fluorescent display device having the CIG structure in which only parts where bonding wires are connected are coated, there is known a device disclosed in Japanese Patent Laid-open Application No. 2003-132824, for example.
As illustrated in FIG. 10, in the fluorescent display device disclosed in Japanese Patent Laid-open Application No. 2003-132824, an IC chip 82 is mounted on a glass substrate 81 by a die bonding paste, and a wiring 83 made of aluminum or the like is formed thereon. Further, each electrode of the IC chip 82 and the wiring 83 corresponding thereto are connected by a bonding wire 84. And also, the electrodes, to which the bonding wires 84 are connected, of the IC chip 82 on the substrate and the bonding wires 84 connected to such electrodes are coated by a coating material 85.
In the fluorescent display devices having such CIG structure, by coating a whole or a part of a front surface of the IC chip 82 with a coating material, it is possible to overcome a malfunction of the IC chip, which is caused by a short-circuit due to a conductive foreign substance connecting the bonding wire 84 with its neighboring wire.
However, in the fluorescent display device of the aforementioned Japanese Patent No. 3553868, a passageway of a gas generated from the die bonding paste for fixing the IC chip 71 is blocked by the coating material 74. Consequently, an adhesive strength of the coating material 74 deteriorates and the IC chip 71 is separated from the glass substrate 51 in worst cases, thereby deteriorating a performance and a reliability of the fluorescent display device.
Further, in the fluorescent display device of Japanese Patent Laid-open Application No. 2003-132824, in order to solve the aforementioned drawbacks, the coating material 85 for coating the electrodes, to which the bonding wires 84 are connected, of the IC chip 82 on the substrate and the bonding wire 84 connected to the corresponding electrode is provided. As a result, the amount of gas generated is reduced by half and, also, the passageway of the generated gas is secured. However, the gas is still discharged from the coating material 85 at the parts where the bonding wires are connected, and the device will face the same problems as those of Japanese Patent No. 3553868.
The coating material used in the aforementioned Patent documents needs to be baked at a temperature higher than 400° C. to 500° C., which leads to a breakdown of the IC chip. Although a coating material in which polyimide resin is dissolved in a solvent is used, such coating material will shrink significantly while being cured during the drying process or the baking process and may develop a crack or a peeling. Further, since the coating material described above is made of oxide of a single particle, the particles in a bulk formed are cured in a most closely packed structure. As a result, there is a problem that a stress is not relaxed and the-crack or the peeling may be developed.
In case the IC chip or the like is coated with a coating material formed of a highly insulating oxide instead of the coating material described above, since most colors of the coating material range from white to dim brown, the coating material is noticeable in the fluorescent display device in which an insulating layer formed of an insulating material such as a lead glass or the like is coated, wherein the lead glass is colored by a paint added with oxide particles of Fe/Cr.